Viewing Events in the Center-of-Mass System

Autor/in	Ruby, Lawrence
Titel	Viewing Events in the Center-of-Mass System
Quelle	In: Physics Teacher, 48 (2010) 2, S.112-113 (2 Seiten)Infoseite zur Zeitschrift PDF als Volltext Verfügbarkeit
Sprache	englisch
Dokumenttyp	gedruckt; online; Zeitschriftenaufsatz
ISSN	0031-921X
DOI	10.1119/1.3293659
Schlagwörter	Kinetics; Physics; Science Education; Theories; Equations (Mathematics); Energy; Teaching Methods; Scientific Concepts; Science Teachers + Suchen Sie Ihr Suchwort? Kinetik; Physik; Naturwissenschaftliche Bildung; Theory; Theorie; Equations; Mathematics; Gleichungslehre; Energie; Teaching method; Lehrmethode; Unterrichtsmethode; Science; Teacher; Teachers; Science teacher; Wissenschaft; Lehrer; Lehrerin; Lehrende
Abstract	In elementary physics, collisions are usually studied by employing the conservation of momentum, and sometimes also the conservation of kinetic energy. However, in nuclear reactions, changes of mass that complicate the situation often occur. To illustrate the latter, we shall cite two examples of endoergic nuclear reactions, i.e., those for which energy must be supplied to make the reaction proceed. A typical situation is given by the equation A + B [arrow right] C + D + Q, (1) where particles A, B, C, and D are expressed in terms of the energy-equivalent of the particle masses, according to the Einstein relation E = mc[squared], and where Q is a negative energy quantity, corresponding to the excess of mass of (C + D) over that of (A + B). Equation (1) is just an alternate statement of the conservation of total energy. Typically, in the lab system (L), energy is supplied as kinetic energy "T" of particle A, and particle B is at rest. Thus, to conserve momentum, particles C and D must compensate for the momentum corresponding to T. Often, it is desirable to know the minimum value of T that will conserve both energy and momentum, i.e., the threshold value of T, known as T[subscript th], that will just allow the reaction to proceed. At threshold, the particles C and D will have their minimum possible kinetic energies. In the center-of-mass system of coordinates (Z) in which the input momentum is zero, at threshold, the products C and D are each stationary, and this requirement will allow us to calculate the corresponding T[subscript th] in the lab system (L). The Z system is often termed the "center-of-mass" system, but it is more properly termed the "zero-momentum" system. (As Provided).
Anmerkungen	American Association of Physics Teachers. One Physics Ellipse, College Park, MD 20740. Tel: 301-209-3300; Fax: 301-209-0845; e-mail: pubs@aapt.org; Web site: http://scitation.aip.org/tpt
Erfasst von	ERIC (Education Resources Information Center), Washington, DC
Update	2017/4/10